METAL PI-COMPLEXES OF BENZENE-DERIVATIVES .50. ARYLBORANES AS SANDWICH LIGANDS - PREPARATION OF [(PRO)(2)B(ETA(6)-PH)](2)M AND [MES(2)B(ETA(6)-PH)](2)M (M=V,CR) - REDOX PROPERTIES AND EPR STUDY OF M(D(5)) NEUTRAL COMPLEXES AND BORON-CENTERED RADICAL-ANIONS

Bis(eta(6)-arene)metal complexes bearing peripheral boryl substituents have been prepared by means of metal-atom ligand-vapor cocondensation leading to [(iPrO)(2)B-eta(6)-C6H5]Cr-2 (7) and [(iPrO)(2)B-eta(6)-C6 H5]V-2 (8) as well as by lithiation and subsequent reaction with Mes(2 )BF with formation of [Mes(2)B-eta(6)-C6H5]Cr-2 (11) and [Mes(2)B-eta( 6)-C6H5]V-2 (12). According to the NMR spectra, rotation about the eta -C-BR(2) bonds in 7 and 11 is free at 25 degrees C, restrictions setti ng in at -85 degrees C. The neutral radicals 8(.) and 12(.) have been studied by EPR spectroscopy: despite of the profound difference in the electron-accepting properties of the groups (iPrO)(2)B- and Mes(2)B-, the hyperfine coupling constants a(V-51) differ by 5% only; the exten t of perturbation of the electronic structure manifests itself more cl early in the g tensor, which is tetragonal for 8 but orthorhombic for 12, the degeneracy of the LUMO's e(1g) being raised in the latter. Cyc lovoltammetry on 11 exhibits reversible oxidation as well as reduction processes. Reduction which, according to EPR, is ligand-centered, occ urs in two steps which are separated by the redox splitting delta E(1/ 2) = 570 mV. This value, which is a measure of electronic communicatio n between the two Mes(2)B groups, lies between those of 1,4-bis(dimesi tylboryl)benzene (delta E(1/2) = 690 mV) and 4,4'-bis (dimesitylboryl) biphenyl (delta E(1/2) = 270 mV). The EPR spectrum of the radical anio n 11(-.) reveals hyperfine coupling to one boron nucleus of a magnitud e very similar to that of the free ligand radical anion Mes(2)BC(6)H(5 )(-.). Therefore, reduction of 11 is ligand-centered, and an intramole cular electron exchange between the boron centers is slow on the EPR t ime scale. As inferred from the observation of the hyperfine interacti ons a(Cr-53) and a(H-1), oxidation of 11 is metal-centered; relative t o parent bis(eta(6)-benzene)chromium (9), the two Mes(2)B groups cause an anodic shift of 290 mV for the couple 11(+/0). The radical cations 11(+.) are prone to protodeborylation, in this aspect resembling the respective silyl derivatives.